High power semiconductor lasers are used to pump cladding pumped fiber lasers. Fiber lasers are capable of producing output power in the multiple kW range and are used in a variety of applications that require high output power such as cutting, welding, material processing (e.g., marking, engraving, and cutting) and directed energy. Achieving the power levels required for these applications is often accomplished by combining the fiber-coupled outputs of multiple diode modules to pump active fibers.
When combining diode pumped fibers it is often convenient to perform the beam combination of the coupled fibers with a fiber based beam combiner that couples a plurality of optical fibers to a single signal fiber. Conventionally, combining multiple fibers to achieve higher power can either reduce optical efficiency or beam quality of the pump fibers. High power (kilowatt-class) fiber pump combiners are vulnerable to small imperfections and losses which have significant impact on reliability.
Commonly-used double clad fibers use a low-index polymer buffer to guide high power pump light in the fiber. When guiding multiple kW of pump power this glass—polymer interface is very sensitive to polymer recoating delamination, defects in the buffer, and contamination, chips, and scratches that occur during the fiber handling and manufacturing process. These defects are a major source of fiber failure in the factory and in the field.
By guiding diode laser pump light in an active fiber with a glass—glass interface, many of the disadvantages of glass/polymer interface can be overcome. In such multi-clad fibers, fluorosilicate glass cladding guides the pump light, making the fiber robust to damage or contamination that occurs during fiber stripping, cleaning, cleaving, splicing and recoating. By guiding the pump light with a glass to glass interface, the buffer avoids the vast majority of interaction with the pump light, thus preventing the gradual degradation or burning that can occur with traditional double clad fiber. One draw-back of glass-clad fibers is that there aren't any developed pump combiners specifically designed for use in glass-clad fibers that leave the fiber core unperturbed.